Download and unpack plugin files to wp-content/plugins/scategory_permalink directory.
Enable “sCategory Permalink” plugin on your Plugins page in Site Admin.
Go to the Options/Permalinks page in Site Admin and use %scategory% option in Custom text field (you can look here for other options). In this blog I’m using /%scategory%/%postname%/.
Now on Write Post page near the categories checkboxes radio button will appear:
Select radio button near category which will be used in permalink.
You could always download latest version of the plugin here.